Hurricanes in the Atlantic are hardly uncommon, but three factors make Sandy
stand out from your run-of-the-mill tropical storm: its size, direction of
travel, and remarkably bad timing.

Sandy is only a “Category 1” storm, meaning it is nowhere near as powerful as its cousin Katrina which devastated New Orleans in 2005, but its hurricane-strength 90mph (144km/h) winds stretch an astonishing 175 miles (289km) from its centre.

A rare combination of weather fronts has seen the hurricane merge with cold air from the north, turning it into a “superstorm” with a total diameter of almost 2,000 miles (3,200km), the largest in Atlantic storm history, experts said.

What makes Sandy almost uniquely dangerous, however, is its projected path which leads from the Atlantic directly into a head-on collision with the coast of New Jersey.

Most tropical storms off the eastern seaboard run directly up the coast but cold air from the north and from the jet stream to the west are forcing Sandy towards the shore.

Professor Mark Saunders, of the Department of Space and Climate Physics at University College London, said: “There is no precedent in hurricane records extending back to 1851 of a storm at this latitude taking this path.”

It means that after reaching the coast Sandy will continue inland and could stretch over 800 miles (1,280km) from the east coast to the Great Lakes.

Dr Adam Lea, of University College London, added: “Pretty much all systems that affect that area are running more or less parallel to the coastline like Irene did last year, so the worst of the wind tends to stay offshore. Because this one is hitting the shore head on it means the worst effects, the highest storm surge and strongest winds, are going to come inland.”

The damage was expected to be particularly severe in New York and parts of New Jersey because they lie just to the right of the projected path of the hurricane’s eye as it meets the shore, where the winds will be strongest.

New York’s exceptionally shallow harbour means Sandy’s storm surge – a gradual rising of the sea level – could be as high as 11ft, which would be the greatest ever seen in the US.

As if to compound the freak nature of the storm, its expected arrival under a full moon meant the storm surge would be swollen by an exceptionally high tide.

Experts added that because the storm is slow-moving and was now being fuelled by the contrast between warm tropical and cold arctic air, it could continue causing damage, flooding or snowfall for up to two days.

Although the combined factors causing the storm were described as a “freak event”, some scientists warned that hurricanes like Irene and Sandy could become more common as climate change raises sea temperatures.

Ross Reynolds, a meteorologist at Reading University, said: “There is some suggestion that as the seas warm you would expect either more cyclones and/or more intense ones over the coming decades.”

But Dr Lea added that the scale of the expected damage was down to “random chance” directing the storm inland and the growing number of people living on the east coast, rather than the direct effect of global warming.